As is well known, in wide band direct sequence CDMA systems, signals consist of different pseudo-random binary sequences that modulate a carrier. Thereby, the spectrum of the signals is spread over a wide frequency range common to a number of channels in the system. Due to the direct sequence coding, orthogonality between signals is achieved, enabling individual decoding of signals from the common frequency range.
This coding principle has many advantages. For instance, direct sequence spread spectrum coding provides substantial reductions of the severity of multi-path fading, which leads to an effective utilization of spectrum resources.
Since signals occupy the same space in the frequency/time domain, an exact power regulation of the individual channels is an important aspect of CDMA systems.
CDMA systems employ power control on both the up- and the downlink. One objective of the power control is to regulate each mobile station transmitter operating within the cell site base-station receiver, such that the signals have the same power level at the base-station receiver regardless of the position or propagation loss of the respective mobile stations. It should be noted that the power level, for each user entity (UE), is proportional to the transmission data rate.
When all mobile station transmitters within a cell site are so controlled, then the total signal power at the base-station receiver is equal to the nominal received power times the number of mobile stations provided that all UEs uses the same data rate.
Each selected signal received at the base-station is converted into a signal that carries the narrowband digital information, whereas the other signals that are not selected constitute a wide band noise signal. However, the bandwidth reduction, which is performed according to the decoding process, increases the signal-to-noise ratio from a negative value to a level that allows operation with an acceptable bit error rate.
The overall system capacity, for instance the number of users that can operate within the cell simultaneously, depends on the minimum signal-to-noise ratio, which produces the given acceptable bit error rate.
The 3rd Generation Partnership Project (3GPP) specification standard for the third generation mobile telephony system supports different user data rates for different users.
According to the W-CDMA specification, an uplink packet data session is performed as follows:
A user entity attempting attaching to the network is informed of the random access channel, RACH, carrier available in respective cells. The user entity transmits information regarding its identity and its present transmission uplink power level. As is well known, the output power of each respective user entity is regulated in a stepwise fashion according to a TPC (transmit power control) loop by the base station, node B, in order to avoid abrupt changes.
Since the RACH channel is shared by up-link traffic from all users, it may become congested due to an overly high load. Consequently, the data throughput may be limited under such circumstances.
Therefore, a given user may be allocated a dedicated packet channel, DPCH, that is exclusive for one user entity according to the specified Hadamard code. DPCH channels are available to a given user entity in respective cells and on respective carriers. Alternatively, a user entity may remain on the RACH channel.
The first message from a user entity, UE, to Node B is always sent on the RACH channel, while subsequent messages may be transmitted on either the RACH channel or a DPCH channel. Hence, if no RACH message can be forwarded, the UE cannot initiate access to the network.
The actual interference level in the cell, the user data rate, the channel quality and the requested quality of the data transmission determine the level of the transmission power needed for a user. The uplink transmission is—especially in large cells—often power limited, i.e. the maximum transmission power is not high enough to reach the desired user data rate or transmission quality.
A common way to determine the uplink cell load is to determine the level of the total received power in comparison to the system noise level. This measure is often called ‘Rise over Thermal noise’ (RoT):
                    ROT        =                              N_T            +                                          ∑                                  UE                  ⁢                                                                          ⁢                  1                                UEn                            ⁢              C                                N_T                                    (        1        )            whereby N_T is the thermal noise in the receiver of the base station.
For the given link it appears thatC_MAX(UEn)=P_MAX(UEn)−L(UEn)  (2)where C_MAX(UEn) is the received maximum power at node B for a user entity transmitting with its maximum allowed power P_MAX at a path loss L, which is mainly dependent on the distance to node B.
The signal to noise ratio of a received signal from a given user entity (e.g. UE1) at node B can be expressed as follows:
                                                                        SIR                ⁡                                  (                                      UE                    ⁢                                                                                  ⁢                    1                                    )                                            =                            ⁢                                                C_MAX                  ⁢                                      (                                          UE                      ⁢                                                                                          ⁢                      1                                        )                                                                    N_T                  +                                                            ∑                                              ≠                                                  UE                          ⁢                                                                                                          ⁢                          1                                                                    UEn                                        ⁢                                          C_MAX                      ⁢                                              (                        UEn                        )                                                                                                                                                                    ≈                            ⁢                                                C_MAX                  ⁢                                      (                                          UE                      ⁢                                                                                          ⁢                      1                                        )                                                                    N_T                  ·                  ROT                                                                                        (        3        )            
The path loss (UE to Node B) can vary substantially depending on the distance between UE and Node B as well as depending on whether the UE is indoor or outdoor. Moreover the isolation between the own cell and the neighboring cell will differ substantially depending on the position of the UE.
Due to the difficulties of estimating path losses and the nature of initial cell planning, it is a problem to allocate spectrum resources efficiently.